An ultrasonic imaging system of the reflex transmission type is shown in U.S. Pat. No. 4,608,868, P. S. Green. The use of an external reflector with such a system is shown in U.S. Pat. No. 4,624,143, P. S. Green. U.S. Patent No. 4,730,495, P. S. Green, discloses means for processing signals from backscatter zones that are not substantially uniform in reflective properties by automatic adjustment of the length of the range zone and weighting of the processed signal accordingly. In U.S. Pat. No. 4,513,749, G. S. Kino, a three-dimensional temperature probe is shown which includes means for measuring local variations in the speed of sound for use in compensating for such sound variations.